When you’re working to understand just how much you’re paying to reach the audience you’re trying to communicate with you might find the CPM metric incredibly insightful. Those insights can help you to determine if your ad is worth running. It can also help you to make comparisons between the performance of your ads on two different platforms.
Facebook calculates CPM by using a few different measurements. The platform takes the total amount you’ve spent and divides it by the number of impressions you generate (multiplied by 1,000.)
If you ran an ad with a 100 dollar budget that gets 10,000 impressions, then your ads have a CPM of $10.